The present invention relates to an exposure method to be used when a mask pattern is transferred onto a substrate such as a wafer in a lithography step for producing a device which includes, for example, a semiconductor element, a liquid crystal display element, a plasma display, and a thin film magnetic head. In particular, the present invention is suitable for an exposure apparatus based on the use of an illuminating apparatus which is provided with an optical system for realizing a uniform illuminance distribution on an illumination objective.
A variety of exposure apparatuses are known, including, for example, projection exposure apparatuses of the full-field exposure type or the scanning exposure type (for example, based on the step-and-scan system), and exposure apparatuses based on the proximity system to be used, for example, when semiconductor elements are produced. Such an exposure apparatus is provided with an illumination optical system for illuminating a pattern on a reticle with a uniform illuminance distribution by using an illumination light beam (exposure light beam) for exposure in order that the minute pattern on the reticle as a mask is highly accurately transferred onto a wafer (or a glass plate or the like) applied with resist, as a substrate.
An optical integrator such as a fly""s eye lens has been hitherto used as an optical member for uniformizing the illuminance distribution. An illumination optical system, which is provided with two-stage fly""s eye lenses (double fly""s eyes) in order to enhance the uniformity of the illuminance distribution, is disclosed, for example, in Japanese Patent Application Laid-Open No. 6-196389 and U.S. Pat. No. 5,636,003 corresponding thereto.
FIG. 17(a) shows main components of a conventional illumination optical system provided with two-stage fly""s eye lenses. With reference to FIG. 17(a), an illumination light beam IL having a width BW1, which is radiated from an unillustrated exposure light source, comes into a first fly""s eye lens 65. Illumination light beams, which come from a plurality of light source images formed on a light-outgoing plane of the first fly""s eye lens 65, come into a second fly""s eye lens 67 via a light-collecting lens system 66. An illumination light beam, which comes from the second fly""s eye lens 67, illuminates a reticle R via a condenser lens system 68.
On the other hand, FIG. 17(b) shows main components of a conventional illumination optical system which has a one-stage fly""s eye lens (single fly""s eye). With reference to FIG. 17(b), an illumination light beam IL, which has a width BW2, comes into a fly""s eye lens 69. Illumination light beams, which come from respective light source images formed on light-outgoing planes of respective lens elements of the fly""s eye lens 69, illuminate a reticle R in a superimposed manner via a condenser lens system 70.
In the case of the double fly""s eye system of the former, the number of the light source images formed in a predetermined direction on the light-outgoing plane of the fly""s eye lens 67 is represented by N1xc2x7N2 provided that N1 and N2 represent the numbers of arrangement of lens elements of the fly""s eye lenses 65, 67 in the predetermined direction. On the other hand, in the case of the single fly""s eye system of the latter, it is necessary that the fly""s eye lens 69 is subdivided so that the number of arrangement of the fly""s eye lens 69 is about N1xc2x7N2 in a predetermined direction in order to obtain the same degree of the uniformity of the illuminance distribution as that of the double fly""s eye system in the predetermined direction.
The exposure light beam has a narrow wavelength width, and it has relatively high coherence (coherency). Therefore, if the exposure light beam is used as it is, then interference fringes which are called speckles are generated on the illumination area of the reticle, and it is feared that any unevenness of the exposure amount is caused thereby. In view of this fact, in order to reduce the temporal coherence of the exposure light beam (or shorten the coherence time) and reduce the interference fringes, Japanese Patent Publication No. 7-104500 (Japanese Patent No. 2071956) discloses a delay optical system in which the exposure light beam is divided into two by using a beam splitter, and two-divided light fluxes are superimposed again after giving a predetermined difference in optical path to them. On the other hand, Japanese Patent No. 2590510 discloses an exposure apparatus in which the temporal coherence is reduced by radiating an exposure light beam onto a beam splitter surface of a delay optical system having a polygonal cross section in which one surface is the beam splitter surface and the remaining surfaces are reflecting surfaces (or total reflection surfaces).
As described above, the conventional exposure apparatus uses the illumination optical system which is provided with the one-stage optical integrator or the two-stage optical integrators. Further, the technique to avoid the occurrence of speckles has been developed. In such circumstances, the exposure wavelength is shortened in an advanced manner in recent years in order to obtain a higher resolution. At present, the KrF excimer laser light beam (wavelength: 248 nm) is dominantly used. In future, investigation will be made to use the vacuum ultraviolet light beam such as the ArF excimer laser light beam (wavelength: 193 nm) and the F2 laser light beam (wavelength: 157 nm). Such a laser beam has high coherence as compared with the conventional bright line. When the laser beam having the short wavelength as described above is allowed to pass through a projection optical system composed of a refractive system, then the usable saltpeter material is limited, for example, to quartz glass and fluorite, and it is difficult to extinguish the color. Therefore, the wavelength of the laser beam is usually narrow-banded, for example, to have an order of the half value width of about 0.1 to 1 pm. The coherence of the laser beam narrow-banded as described above is further enhanced, resulting in high contrast of the interference fringes (speckles). Therefore, it is necessary to use a more highly-advanced technique in order to avoid the occurrence of interference fringes.
Recently, the areal size per one chip of the semiconductor element is increased. Further, it is also effective to increase the numerical aperture of the projection optical system in order to obtain a higher resolution. However, it gradually becomes difficult to design and produce a projection optical system with which high image formation performance is successfully obtained over an entire exposure area that is large and wide. In view of the above, the attention is attracted to a scanning exposure type projection exposure apparatus in which exposure is performed by synchronously moving a reticle and a wafer with respect to a projection optical system in a state in which an exposure light beam is radiated into a slender slit-shaped illumination area on the reticle. In this case, the narrower the width of the illumination area in the scanning direction (hereinafter referred to as xe2x80x9cslit widthxe2x80x9d) is, the wider the width of the exposure field is, when a projection optical system having an identical size is used. As a result, it is possible to perform the exposure for a chip pattern having a large areal size. However, when the pulse light beam such as the excimer laser light beam is used, it is necessary that the number of exposure pulses for each point on the wafer is not less than a predetermined minimum number of pulses, taking the dispersion or irregularity of the pulse energy into consideration. When the movement velocity of the stage is increased in order to enhance the throughput in a state in which the slit width is made narrow, the number of exposure pulses is consequently decreased. However, it is possible to increase the frequency of the pulse light emission in the case of the recent excimer laser light source. The problem of the number of exposure pulses has been progressively dissolved.
However, when the slit width is narrowed, if a fly""s eye lens is used as the optical integrator, then it is necessary to use finely subdivided lens elements in the scanning direction to such an extent corresponding to the narrowed slit width. When the arrangement pitch of a plurality of lens elements is made fine as described above, the interference fringes tend to occur in the illumination area of the reticle, because the coherence of the light fluxes passing through the adjacent lens elements is enhanced.
That is, in the case of the double fly""s eye system shown in FIG. 17(a), the width of each lens element of the fly""s eye lens 65 at the first stage can be wider than xcex94 provided that xcex94 represents the coherence length in the lateral direction of the illumination light beam IL. In this case, it is assumed that the light fluxes A1, A2, which come into positions in the vicinity of the interface between certain adjacent lens elements of the fly""s eye lens 65, come into the different points P1, P2 on the reticle R respectively. It is also assumed that the light fluxes B1, B2, which come into positions separated from the boundary between the lens elements by spacing distances xcex941, xcex942 (assuming that xcex941+xcex942=xcex94 is satisfied) respectively, also come into the points P1, P2 respectively via the fly""s eye lens 67. Interference occurs at the points P1, P2, respectively and interference fringes are formed on the reticle R.
Similarly, in the case of the single fly""s eye system shown in FIG. 17(b), it is assumed that the light fluxes A3, A4, which come into a certain boundary between the lens elements, come into the different points P3, P4 on the reticle R respectively, and the light fluxes B3, B4, which come into a boundary adjacent thereto, also come into the points P3, P4 respectively, provided that the lens element of the fly""s eye lens 69 has the width which is formed to be narrow in the same degree as that of the coherence length xcex94 in the lateral direction of the illumination light beam IL. Also in this case, interference occurs at the points P3, P4, respectively and interference fringes are formed on the reticle R.
That is, it is necessary to use the optical integrator in order to realize the uniform illuminance distribution on the reticle of the laser beam in which the illuminance distribution has a shape of Gaussian distribution. However, if the reticle is illuminated in a superimposed manner by using the optical integrator, the interference fringes tend to appear. An exposure apparatus has been also developed, in which a rod-type integrator (rod lens) is used as the optical integrator. However, the conventional rod-type integrator involves such an inconvenience that the interference fringes tend to appear in the same manner as in the fly""s eye lens.
In view of the above, in order to decrease the temporal coherence and decrease the interference fringes, it is also possible to use the delay optical system or the delay optical element as described above in combination. However, the conventional delay optical system and the delay optical element have such a tendency that the size is increased and the weight is increased, for the recent exposure light beam composed of the laser beam which is narrow-banded and which has the high coherence. If the delay optical system or the delay optical element is allowed to have a function to uniformize the illuminance distribution as well, it is possible to simplify the arrangement of the illumination optical system.
In order to mitigate the uneven illuminance caused by the interference fringes without using the delay optical system or the delay optical element, for example, the following method has been hitherto used as well. That is, a vibration mirror is arranged in front of the fly""s eye lens, and the laser beam, which comes into the fly""s eye lens, is vibrated to move the interference fringes on the reticle. By doing so, the uneven illuminance is reduced by means of the integrating effect. In this case, the control is made so that the interference fringes are gradually moved every time when the pulse light emission is performed, because the excimer laser or the like resides in the pulse light beam. However, in the case of the method based on the use of the vibration mirror as described above, it is necessary to ensure a certain degree of exposure time. For this reason, an inconvenience arises as follows. That is, if it is intended to obtain necessary uniformity of the exposure amount distribution, then the exposure time is prolonged, and the throughput is lowered.
Taking the foregoing points into consideration, a first object of the present invention is to provide an exposure method in which a substantially uniform illuminance distribution is obtained on a pattern of a transfer objective without complicating an illumination optical system so much, without increasing the size of the illumination optical system so much, and without prolonging the illumination time (exposure time), even when an illumination light beam (exposure beam or illumination beam) having high coherence is used.
A second object of the present invention is to provide an illuminating apparatus which can be used when the exposure method as described above is carried out.
A third object of the present invention is to provide an exposure apparatus which makes it possible to perform exposure with a high throughput and with small unevenness of exposure amount by using the illuminating apparatus as described above.
A fourth object of the present invention is to provide an exposure method which makes it possible to improve uniformity of totalized exposure amount distribution on an exposure objective substrate after scanning exposure without increasing the size of an illumination optical system so much, when the scanning exposure is performed by using a pulse light beam (exposure beam or illumination beam) having high coherence.
A fifth object of the present invention is to provide an illuminating apparatus or an exposure apparatus which can be used when the exposure method as described above is carried out.
A first exposure method according to the present invention resides in an exposure method for illuminating a first object (R) with an exposure light beam to transfer a pattern on the first object onto a second object (W), the exposure method comprising adjusting the exposure light beam into light fluxes having a predetermined angular aperture distribution, and allowing the adjusted light fluxes to pass through a substantially closed loop-shaped optical path so that a plurality of light fluxes, which have passed through the loop-shaped optical path a variety of numbers of times depending on angular apertures respectively, are superimposed and guided to the first object.
According to the present invention as described above, when the exposure light beam, which has the predetermined angular aperture or open angle distribution, is supplied to the loop-shaped optical path, the components of the exposure light beam, which have different angular apertures or open angles (angles of incidence), are advanced while repeating reflections at the outer circumferential surfaces of the optical path depending on the angular apertures respectively. In this operation, for example, when a window, which is smaller than the cross section of the optical path, is formed at an intermediate position of the optical path beforehand, the components, which pass through the window, are radiated toward the first object. On the other hand, the components, which are spread widely beyond the window, pass through the optical path again, and the components, which pass through the window, are finally radiated toward the first object. As a result, the components, which have passed through the loop-shaped optical path once, twice, three times or more depending on the angular apertures respectively, are superimposed and radiated from the window.
When the optical path length, which is required for the light flux to pass through the loop-shaped optical path once, is set to be longer than the coherence length that is determined depending on the coherence time, the coherence between the plurality of components is greatly lowered owing to the delay effect. That is, the temporal coherence of the exposure light beam radiated from the window is lowered owing to the delay effect depending on the angular apertures of the respective components, and the spatial coherence thereof is also lowered. Further, the information on the angular aperture (angle of incidence) upon the incidence is maintained by the reflection at the outer circumference. Further, the illuminance distribution is uniformized as well by the repeated reflection at the outer circumference. In other words, the light flux is divided (subjected to wave front division) depending on the size of the cross section of the loop-shaped optical path and the size of the window for radiation. Therefore, the delay effect and the uniformizing effect of the illuminance distribution are obtained owing to the loop-shaped optical path. Accordingly, even when the exposure light beam has a Gaussian distribution with high coherence, then the interference fringes (speckles) on the pattern of the transfer objective are reduced on condition that the illumination optical system is simplified, and it is possible to obtain a substantially uniform illuminance distribution.
In another aspect, a first illuminating apparatus of the present invention resides in an illuminating apparatus for illuminating a pattern on an illumination objective (R) with an illumination light beam from a light source (15), the illuminating apparatus comprising an optical member (22) including a window (44) which receives the illumination light beam from the light source, wherein a plurality of light fluxes, which are obtained by allowing light fluxes incoming from the window to pass through the optical member (22) a variety of numbers of times depending on angular apertures respectively, are superimposed and radiated toward the illumination objective. According to the present invention as described above, the optical member (22) forms a loop-shaped optical path to act as a delay optical element for the illumination light beam and an element for uniformizing the illuminance distribution. Therefore, it is possible to use the first exposure method according to the present invention.
In this arrangement, it is desirable that an angular aperture-adjusting optical system (21) for adjusting the illumination light beam from the light source into light fluxes having a predetermined angular aperture distribution is arranged between the light source and the optical member; and a multiple light source-forming optical system (25) for forming a plurality of light source images from the illumination light beam from the optical member, and a condenser optical system (7) for radiating light fluxes from the plurality of light source images onto the illumination objective in a superimposed manner are arranged between the optical member and the illumination objective.
It is desirable that the window (44) is arranged at a position which is eccentric from a central axis of the optical member (22). It is preferable that the window also serves as a window for radiating the illumination light beam. Examples of the optical member include those obtained by arranging, in a ring-shaped configuration, outer surface reflection members (for example, mirrors) or prism-shaped or columnar transmitting members (for example, rod members).
When the window is eccentric from the central axis of the optical member, the incoming illumination light beam arrives at the reflecting surfaces under different conditions in the upward, downward, rightward, and leftward directions. As a result, the delay condition (for example, the number of times of passage through the loop), which is determined depending on the angular aperture, is mutually different in the upward, downward, rightward, and leftward directions. The delay condition for the illumination light beam advancing in an oblique direction is a condition obtained by averaging the delay conditions in the upward and downward directions and in the rightward and leftward directions in the vicinity thereof. As a result, one optical member can be used as a large number of delay optical systems having different delay conditions. The temporal coherence is further lowered.
Besides the window is eccentric from the central axis of the optical member as described above, it is also preferable that the direction of the illumination light beam supplied to the window is allowed to have a predetermined inclination with respect to the central axis.
In still another aspect, a first exposure apparatus according to the present invention resides in an exposure apparatus for transferring a pattern on a first object onto a second object, wherein the pattern on the first object is illuminated with an illumination light beam from the illuminating apparatus of the present invention.
In still another aspect, a second exposure apparatus according to the present invention resides in an exposure apparatus provided with an illumination system for illuminating a first object (R) with an exposure light beam in which a second object (W) is exposed with the exposure light beam via the first object; the exposure apparatus comprising an optical member (22) which includes a transmitting section (22a to 22h) for internally reflecting the exposure light beam in the illumination system and changing a traveling direction thereof; wherein the optical member is formed with an aperture (44) which is smaller than a cross-sectional area of the transmitting section in order to radiate the exposure light beam. When the exposure apparatuses as described above are used, then interference fringes are scarcely formed on the first object, and the exposure amount distribution is uniformized. Accordingly, it is possible to improve the line width uniformity of the pattern to be transferred onto the second object.
In still another aspect, a second exposure method according to the present invention resides in an exposure method for illuminating a first object (R) with an exposure light beam to expose a second object (W) with the exposure light beam having passed through a pattern on the first object; the exposure method comprising introducing the exposure light beam into a plane (76G) which is substantially conjugate with a pattern plane of the first object via an open light-feeding optical path (56) which is surrounded by reflecting surfaces and which has at least one bent section (58A), and introducing, into the first object, the exposure light beam having passed through the plane.
According to the present invention as described above, a plurality of light source images (secondary light sources) are formed as virtual images in accordance with the reflection at external surfaces of the light-feeding optical path in the same manner as in a rod-type integrator. In this procedure, the coherence is lowered for the adjacent light source images, because they are inverted in relation to the reflecting surfaces. As a result, the contrast is lowered for interference fringes (speckles) generated on the first object. The formation of the plurality of light source images can be considered as wave front division of the exposure light beam as well. When the angular aperture of the exposure light beam (coherence factor of the illumination system) is increased, or when the light-feeding optical path is lengthened, then the number of formed light source images (number of wave front division) is increased, the integration effect is enhanced, and the coherence is decreased as well. Therefore, it is possible to improve the uniformity of the illuminance distribution on the pattern of the first object. However, when the light-feeding optical path is merely lengthened, the illumination optical system inevitably has a large size. However, in the present invention, the light-feeding optical path is bent. Therefore, the illumination optical system can be miniaturized.
In this arrangement, for example, when a laser beam, in which the oscillation wavelength width is narrow-banded to have an order of about 0.1 to 1 pm, is used, it is desirable that the light-feeding optical path is provided with at least three bent sections, in order that the light-feeding optical path is sufficiently long to obtain a sufficiently uniform illuminance distribution on the pattern of the first object, and the illumination optical system is allowed to have a small size.
It is desirable that a width of the light-feeding optical path (56) on a light-outgoing side (57B) is wider than a width of the light-feeding optical path (56) on a light-incoming side (57A) in a bending direction brought about by the bent section (58A). One method for increasing the number of wave front division as described above is to increase the angular aperture of the exposure light beam. However, if the angular aperture is increased in a state in which the cross-sectional area of the light-feeding optical path is constant, the light amount loss is increased at the bent section. On the contrary, when the width d2 on the light-outgoing side is made wider than the width d1 on the light-incoming side of the light-feeding optical path, it is possible to decrease the light amount loss at the bent section.
In still another aspect, a third exposure method according to the present invention resides in an exposure method for illuminating a first object (R) with a pulse-emitted exposure light beam and synchronously moving the first object and a second object (W) to perform scanning exposure for the second object with the exposure light beam having passed through a pattern on the first object; the exposure method comprising previously measuring a repeating pitch (Q1) of an intensity distribution of the exposure light beam on the second object in a scanning direction for the second object; and setting a distance (Q2) of movement of the second object in the scanning direction during one cycle of pulse light emission of the exposure light beam to be a non-integral multiple of the measured pitch.
In the present invention as described above, even if any unevenness (interference fringe or the like) of the intensity distribution having the pitch Q1 is generated on the second object in the scanning direction for the second object by the exposure light beam in an amount of one pulse, the exposure light beam is subjected to the pulse light emission every time when the second object is moved in the scanning direction by the distance Q2 (xe2x89xa0nxc2x7Q1, n is an integer of not less than 1). Accordingly, the distribution of the totalized exposure amount on the second object is gradually uniformized owing to the averaging effect.
In still another aspect, a second illuminating apparatus according to the present invention resides in an illuminating apparatus for illuminating a pattern on an illumination objective (R) with an illumination light beam from a light source (9); the illuminating apparatus comprising a multiple light source-forming optical system (56) which includes a plurality of transmitting sections (57A to 57G) surrounded by reflecting surfaces respectively for allowing the illumination light beam to pass through an interior thereof, and one or a plurality of reflecting section or sections (58A to 58F) for bending an optical path for the illumination light beam at a boundary between the plurality of transmitting sections, in which the illumination light beam is incorporated at one transmitting section (57A) of the plurality of transmitting sections, and the illumination light beam is radiated from another transmitting section (57G) onto a plane (76G) that is substantially conjugate with a pattern plane of the first object; and a condenser optical system (31, 7) which collects, onto the pattern, the illumination light beam having passed through the substantially conjugate plane.
In still another aspect, a third exposure apparatus according to the present invention resides in an exposure apparatus comprising the second illuminating apparatus of the present invention which illuminates a first object (R) as an illumination objective with an illumination light beam from the illuminating apparatus; wherein a second object (W) is exposed with the illumination light beam having passed through a pattern on the first object.
In still another aspect, a fourth exposure apparatus according to the present invention resides in an exposure apparatus having an illumination system for illuminating a first object (R) with an exposure light beam, for exposing a second object (W) with the exposure light beam via the first object; the exposure apparatus comprising an optical member (56) which includes a transmitting section (57A, 58A, 57B) for internally reflecting the exposure light beam in the illumination system; wherein the transmitting section of the optical member is bent at least at one position, and a width (width d2) of the transmitting section after being bent is larger than a width (width d1) of the transmitting section before being bent.
In still another aspect, a fifth exposure apparatus according to the present invention resides in an exposure apparatus for illuminating a first object with an exposure light beam from a pulse light source (9) and synchronously moving the first object and a second object by the aid of a stage system (1, 4, 5) to perform scanning exposure for the second object with the exposure light beam having passed through a pattern on the first object; the exposure apparatus comprising a storage unit (13a) which stores a repeating pitch of an intensity distribution of the exposure light beam on the second object in a scanning direction for the second object; and a control system (13) which controls a light emission frequency of the pulse light source and a scanning velocity of the second object effected by the stage system depending on the stored pitch.
The second exposure method and the third exposure method can be carried out by using the second illuminating apparatus and the third and fourth exposure apparatuses and the fifth exposure apparatus of the present invention respectively.
It is desirable for the fifth exposure apparatus to provide a photoelectric detector (2) for measuring the intensity distribution on the stage for driving the second object.
In still another aspect, a method for producing a device according to the present invention comprises the step of transferring a device pattern (R) onto a workpiece (W) by using the exposure method of the present invention as described above. When the present invention is applied, the unevenness of the exposure amount is reduced. Therefore, it is possible to produce a device having a highly advanced function.